Various techniques are conventionally proposed in which a swirl is generated in a combustion chamber during a low-load operation of an internal combustion engine to enhance the combustion efficiency.
For example, an intake part structure is described in Japanese Utility Model Application Laid-open No. 158226/80, in which a projection is provided on an inner wall of an intake port in order to deflect a flow of intake air into a tangential direction of a cylinder bore to generate a swirl.
In addition, an intake port structure is described in Japanese patent Publication No.32483/86, in which a semi-spherical recess is provided in an inner wall of an intake port in order to deflect a flow of intake air into a tangential direction of a cylinder bore to generate a swirl.
Also, an intake port structure is described in Japanese Utility Model Application Laid-open No. 179240/84, in which two juxtaposed intake ports are connected by a communication passage which deflects a flow of intake air into a tangential direction of a cylinder bore to generate a swirl.
All of the conventionally proposed structures primarily aim at increasing the speed component of the intake air flowing into the combustion chamber in the direction of the tangential direction of the cylinder bore. However, all of these structures suffer from a problem that if the speed component in the tangential direction of the cylinder bore is large, i.e., if the intake air flows in a direction to collide against a top face of the piston, the speed component in the tangential direction is decreased, so that a sufficient swirl is not generated.